The present invention relates to the field of optics, and in particular to the enhanced transmission of light through a metal film perforated with one or more apertures.
As discussed in detail in U.S. Pat. No. 6,236,033 to Ebbesen et al., the optical transmission of a single, sub-wavelength aperture in a metal film can be strongly enhanced when the incident light is resonant with surface plasmons at the film""s surface. In order to couple the incident radiation to the surface plasmons, the surface of the metal film must be provided with a periodic surface topography (i.e. surface features such as dimples or protrusions). The transmission enhancement has great potential in applications which require high optical throughput at high resolution, such as near-field microscopy and high-density optical data storage. This effect has the attractive feature that the wavelength of the resonance can be tuned by the periodicity of the surface features, the angle of incidence of the incident light and the refractive index of a dielectric provided substantially adjacent to the film. See, e.g., U.S. Pat. No. 5,973,316 to Ebbesen et al., U.S. Pat. No. 6,040,936 to Kim et al., U.S. Pat. No. 6,052,238 to Ebbesen et al., U.S. Pat. No. 6,236,033 to Ebbesen et al., and U.S. Pat. No. 6,285,020 to Kim et al. (each of these patents being incorporated herein by this reference and being referred to herein as the xe2x80x9cPrevious Patentsxe2x80x9d).
The inventors have recently studied the effect of surface feature geometry on transmission enhancement, and found that the highest enhancement occurs for a set of concentric circular grooves (also referred to as depressed rings) surrounding a central, sub-wavelength aperture, in which the optimal groove depth is a few times the skin depth of the metal film. See T. Thio et al., xe2x80x9cEnhanced Light Transmission Through a Single Subwavelength Aperture,xe2x80x9d Optics Letters, vol. 26, no. 24, pp. 1972-1974 (2001).
It has also been found that the an optimal periodic surface topography configuration is one in which the diameter of the surface features is one-half the periodicity of the surface features (that is, where dSF=P/2), and in such a configuration, the optimal aperture diameter is dA=dSF=P/2. However, as the diameter of the aperture is made smaller, the transmission enhancement is reduced.
What is needed is an enhanced optical transmission apparatus which provides useful transmission enhancement even with very narrow apertures.
The present invention is an apparatus for enhanced light transmission wherein the transmission efficiency is even further enhanced in comparison to prior art devices by optimizing the geometry of the aperture as well as the relationship between the geometry of the aperture and the geometry of the surrounding periodic surface topography.
Generally speaking, an apparatus for enhanced light transmission is provided. The apparatus comprises a metal film having a first surface and a second surface, at least one aperture being provided in the metal film and extending from the first surface to the second surface. The at least one aperture comprises an entrance portion disposed on the first surface of the metal film and an exit portion disposed in the second surface of the metal film, each portion having a cross-sectional area in the plane of the corresponding metal film surface, wherein the cross-sectional area of the entrance portion is not equal to the cross-sectional area of the exit portion. A periodic surface topography is provided on at least one of the first and second surfaces of the metal film, the periodic surface topography comprising a plurality of surface features, wherein the geometry of each aperture entrance portion substantially matches the geometry of the surface features.
The present invention provides optical transmission enhancement even with apertures having an exit portion diameter which is significantly less than half the periodicity of the surface features.